A series of three replicated 3 × 3 Latin-square design experiments was carried out with lactating dairy cows (15 in experiments 1 and 3, and 18 in experiment 2). The objectives of these experiments were to investigate if the feeding of high concentrations of molasses would be detrimental to the performance or health of cows, and if the strategic supplementation of nitrogen or unprotected fat could enhance the efficiency of molasses feeding. In experiment 1, complete diets were based on grass silage with three inclusion rates of a liquid molasses-based supplement (Molaferm 20) to supply molasses dry matter (DM) of 125, 250 and 375 g/kg DM respectively. In experiment 2, the control diet contained (g/kg DM) 460 grass silage, 310 'molaferm 20' and 192 barley and 22 soya-bean meal, and was then supplemented with urea (12 g/kg DM) or soya-bean meal (177 g/kg DM). In experiment 3, the control diet was based on (g/kg DM) grass silage (460), molaferm 20 (310), barley (130), soya-bean meal (65) and oatfeed (20). Two concentrations, low and high, of unprotected tallow were added to the control diet (12·5 and 25·8 g/kg DM, respectively).
In experiment 1, cows suffered from some scouring on the highest molasses treatment, but recovered when changed to either the medium or low molasses treatment. However, there were no clinical symptoms of ill health observed during experiments 2 and 3. In experiment 1, animal intake and performance on the diets containing molasses DM of 125, 250 and 375 g/kg DM respectively were: total DM intake, 12·8, 16·2 and 18·6 kg/day (s.e.d. 1·03, P < 0·001); milk yield, 15·5,17·4 and 17·6 kg/day (s.e.d. 0·53, low v. medium or high, P < 0·01); fat concentration, 39·6, 39·7 and 40·0g/kg (s.e.d. 0·70, P>0·05); protein concentration, 31·6, 32·7 and 33·6g/kg (s.e.d. 0·22, P < 0·05). In experiment 2, supplementation of the control diet both with urea and soya-bean meal significantly increased DM intake (P < 0·01); milk yield (P < 0·01) and protein concentration in milk (P < 0·05), but had no significant effects on fat concentration in milk (P > 0·05). In experiment 3, there were no significant differences in DM intake between the three treatments. However, supplementation with both low and high concentrations of unprotected tallow significantly increased milk yield (P < 0·01) and significantly decreased fat (P < 0·01) and protein (P < 0·001) concentrations in milk.
It is concluded that lactating dairy cows could be given molasses up to 250 g/kg DM in grass silage-based complete diets without adverse effects on milk production or health. When a diet contained a high concentration of molasses (248 g/kg DM), supplementation with urea and soya-bean meal increased intake and milk production, and supplementation with unprotected tallow also increased milk yield, but fat and protein concentrations in milk were decreased.